Spot beam location method and apparatus

ABSTRACT

A mobile terminal ( 6 ) communicates via a satellite ( 8 ), which projects a number of overlapping spot beams ( 10 ), with a fixed earth station ( 2 ). To determine which spot beam ( 10 ) is to be used for communication, the mobile terminal signals in a common signalling channel which is simultaneously received by all, or a subgroup of, the spot beams ( 10 ). The satellite ( 8 ) retransmits to the fixed earth station ( 2 ) in separate channels the signal as received by each of the spot beams ( 10 ) which are able to receive the common signalling channel. The relative strengths of the signal as received in the different spot beams ( 10 ) are compared at the fixed earth station and a communications channel is assigned to the mobile terminal in one of the spot beams ( 10 ), selected according to the comparison of signal strengths.

TECHNICAL FIELD

The present invention relates to a method and apparatus for radiofrequency communications, and in particular to a method and apparatusfor selecting a spot beam for communication with a radio frequencytransceiver in a satellite communications system in which a satelliteprojects a plurality of overlapping spot beams over an area of theearth's surface. Each spot beam carries a plurality of communicationchannels, which are re-used in spot beams which do not overlap. In thisway, the communications traffic capacity of a satellite communicationsystem is greatly increased over satellite systems which only use asingle beam per satellite.

BACKGROUND ART

However, a problem associated with satellites having multiple spot beamsis that, in order to communicate with a transceiver, it is necessary toknow in which spot beam the transceiver is located, so that anappropriate communications channel can beassigned to the transceiver inthat spot beam. This problem arises both for geostationary satellites,because the transceivers may move between and during calls, and fornon-geostationary satellites, because the satellites move between andduring calls as well.

One solution to this problem is described in the document WO 93/09613,in which each spot beam carries a “pilot tone” which identifies thatbeam. A terrestrial terminal must scan through all of the differentchannels on which pilot tones are transmitted, and send a signal back tothe satellite when a pilot tone is received. However, the requirement toscan a large number of channels leads to delay, and requires complicatedcircuitry and high power consumption in the terrestrial terminal.Moreover, the pilot tones occupy channels which could otherwise be usedfor communication and are wasteful of the frequency spectrum.

The document GB-A-2 275 588 discloses a similar system, in whichidentification information is transmitted in each spot beam. A mobileterminal receives the information and transmits the information back viathe satellite to a terrestrial network, which registers the locationinformation. To avoid interference between spot beams, theidentification information must be transmitted in different channels ineach spot beam. This system therefore incurs a similar problem to thatin the system described in WO 93/09613.

STATEMENT OF INVENTION

According to one aspect of the present invention there is provided amethod of selecting an appropriate one of a number of spot beamsprojected by satellite for communication with a terrestrial terminal, inwhich the spot beams share a return signalling channel. The terrestrialterminal sends a signal in this signalling channel, and the location ofthe terrestrial terminal is determined by comparing the signal asreceived in each of the spot beams.

In another aspect of the present invention, there is provided asatellite which has a multi-beam antenna for communication with aterrestrial terminal. The satellite is arranged so that a signallingchannel can be received in all the spot beams constructed by the multibeam antenna. The satellite maps the signalling channel received in eachspot beam generated by the multi-beam antenna to a plurality of channelsin the feeder link to a base station, each of these channels in thefeeder link corresponding to the signalling channel received in one ofthe spot beams.

An advantage of the above aspect of the present invention is that theterrestrial terminal does not need to search a wide range offrequencies, but need only send a signal in the signalling channel. Thelocation of the terrestrial terminal may then be determined at the basestation and an appropriate communications channel may be allocated. Thismethod simplifies the operation of the terrestrial terminal and reducesits power consumption. Moreover, since only one signalling channel isused for all the spot beams, more channels can be allocated forcommunications.

According to another aspect of the present invention, there is provideda method of selecting one of a number or spot beams projected by asatellite for communication with a terrestrial terminal, in which atleast three return signalling channels are allocated to the spot beamsin such a way that the same signalling channel is not allocated tooverlapping spot beams. A terrestrial terminal transmits a signal ineach of the signalling channels, and the signal received in each of thesignalling channels is compared to determine in which spot beam theterrestrial terminal is located.

Although this method requires that the terrestrial terminal transmit inthree signalling channels, it can be advantageously applied to existingsatellites which cannot be arranged to assign the same channel toadjacent spot beams.

The average time taken for the terrestrial terminal to indicate itsposition may be reduced by transmitting a signal to the terrestrialterminal to prevent it from further signalling as soon as the locationof the terrestrial terminal is identified. Thus, if the terrestrialterminal is located in a spot beam to which the first signalling channelis assigned, the terrestrial terminal need not proceed to signal in thesecond and third signalling channels.

Preferably, the satellite is able to project a wide beam whichsubstantially covers the area of all the spot beams, and information isbroadcast in the wide beam which identifies which signalling channelsmay be used. The terrestrial terminals receive this information andtransmit signals on the signalling channel or channels indicated by thisinformation. In this way, the signalling channels may be flexiblyassigned, and calls may be placed to the terrestrial terminals bybroadcasting a call request in the wide beam. Moreover, the terrestrialterminals may be used to communicate via different satellites to whichdifferent signalling channels are assigned.

Once the location of the terrestrial terminal is determined, acommunications channel may be assigned to the terrestrial terminalwithin an appropriate spot beam. The appropriate spot beam may be thespot beam in which the strongest signal in the signalling channel orchannels was received. Alternatively, where the signal was received inmore than one soot beam, a communications channel may be allocated tothe terrestrial terminal according to the level of existingcommunications traffic in each of the spot beams in which the signal wasdetected. Alternatively, where the satellite is a non-geostationarysatellite, a communication channel may be assigned to a spot beam whichis approaching the terrestrial terminal.

According to another aspect of the invention, there is provided a methodof selecting one of a plurality of cells for communication with a radiofrequency transceiver in a cellular communications system, in whichsignalling channel information is is broadcast over the area of theplurality of cells, the transceiver transmits a signal in a signallingchannel indicated by the signalling channel information and one of theplurality of cells is selected for communication with the radiofrequency transceiver on the basis of a quality of the signal receivedin the signalling channel.

The present invention also extends to apparatus for carrying out any ofthe methods described above.

BRIEF DESCRIPTION OF DRAWINGS

Specific embodiments of the present invention will now be described withreference to the accompanying drawings, in which:

FIG. 1 shows a satellite communications system including multipleoverlapping spot beams;

FIG. 2 is a block diagram of the communication sub-system of thesatellite of FIG. 1;

FIG. 3 is a schematic block diagram of a mobile terminal for use withthe communications system of FIG. 1;

FIG. 4 is a schematic block diagram of a land earth station for use withthe satellite communications system of FIG. 1;

FIG. 5 is a protocol diagram of call set-up initiated by the mobileterminal in a first embodiment of the present invention;

FIG. 6 is protocol diagram of call set-up initiated by the land earthstation in the first embodiment;

FIG. 7 is a protocol diagram of call set-up initiated by the mobileterminal in a second embodiment of the present invention;

FIG. 8 is a protocol diagram of call set-up initiated by the land earthstation in the second embodiment;

FIG. 9 is a protocol diagram of call set-up in a first variant of thesecond embodiment; and

FIG. 10 is a diagram of a cellular communications system incorporating athird embodiment of the present invention.

MODES FOR CARRYING OUT INVENTION

FIG. 1 shows a satellite communications system such as the INMARSAT-B™,INMARSAT-M™, INMARSAT-C™, or INMARSAT MINI-M™ systems, using a satellite8 such as, for example, the INMARSAT 3 Satellite.

A land earth station (LES) 2 is connected to a terrestrial network, suchas a PSTN, and provides an interface between the terrestrial network anda feeder link 4 to the satellite 8. The satellite 8 projects seven spotbeams 10 a to 10 g on the earth's surface. An alternative number of spotbeams, such as five spot beams, may be used. The spot beams 10 are usedfor both forward and return communications traffic. A mobile earthstation (MES) 6 is assigned a communications channel within the spotbeam 10 in which it is located. The satellite 8 also projects a globalbeam 11 which encompasses all the spot beams 10.

In this embodiment, the satellite 8 is a geostationary satellite, andtherefore the MES 6 does not generally move from one spot beam 10 toanother during a call. However, the MES 6 may be used within the wholearea covered by the satellite 8 and may be moved to another spot beam 10between calls.

The operation of the satellite 8 will be explained in greater detailwith reference to FIG. 2. In the communications sub-system of thesatellite 8, a C-band receive antenna 12, which is directed towards theLES 2, receives forward-link channels from the LES 2. The forwardchannels are down-converted from the C-band to the L-band by a C-bandreceiver 14, and the L-band output is passed to a forward intermediatefrequency processor 16 which divides the received spectrum into separatebeam outputs. The separate beam outputs are input to a beam formingmatrix 18, which generates outputs for individual, elements of amulti-beam L-band transmit antenna which generates the spot beams 10 andthe global beam 11.

The return part of the communication sub-system comprises an L-bandreceive antenna 22 which has an array of receiving elements. The outputsfrom the receiving elements are routed to a return combiner 24 whichconverts the array element outputs to spot beam outputs and a globalbeam output. The return beams are substantially co-terminous with thecorresponding forward beams. The beam outputs from the return combiner24 are passed to a return intermediate frequency processor 26 whichallocates beam outputs to corresponding parts of the C-band spectrum.The output of the return intermediate frequency processor 26 isup-converted to the C-band in a C-band transmitter 28, and transmittedto the LES 2 through a C-band transmit antenna 30. The frequency bandsused by the satellite 8 are mentioned purely by way of example and anyfrequency bands suitable and available for satellite communications maybe used.

The arrangement of the LES 2 is shown in more detail in FIG. 3. The LES2 is connected to a PSTN 32 by several lines, to allow voice or datacommunication with multiple users connected to the PSTN 32. Incomingsignals from the PSTN 32 are connected to a forward PSTN interface 34which demodulates data in the case of a data call, and encodes audiosignals in the case of a voice call. The forward PSTN interface 34outputs a plurality of channels of digital data, each corresponding toone call connected to the PSTN 32, to a radio frequency modulator 36which modulates the data from each call into a corresponding frequencychannel in the C-band. The radio frequency modulated signal istransmitted through an antenna 40 to the C-band receive antenna 12 onthe satellite 8.

Signals transmitted by the C-band transmit antenna 30 of the satellite 8are received by the antenna 40 connected to the LES 2. Each C-bandfrequency channel is demodulated by a radio frequency demodulator 42 toform a separate stream of digital data. Each digital data stream ismodulated by a return PSTN interface 44 to generate signals suitable fortransmission over the PSTN 32, with each data stream being assigned to adifferent line of the PSTN 32. The operation of the forward and returnPSTN interfaces 34 and 44 is controlled by a controller 46, whichdetects the status of each call and, in the case of data call, emulatesdata circuit terminating equipment with respect to the PSTN 32.

As described above, the LES 2 converts multiple voice and/or data callsconnected to the PSTN 32 into corresponding frequency channels in thefeeder link 4. The allocation of calls to frequency channels iscontrolled by access control and signalling equipment (ACSE) 48, whichcommunicates with a network control station NCS (not shown) to negotiatethe allocation of communications channels through the satellite.

FIG. 4 shows the arrangement of the MES 6 in greater detail. The MES 6is connected to an antenna 50 for transmitting signals to the satellite8 and receiving signals therefrom. The antenna 50 is connected to aradio frequency modulator/demodulator 52, which converts L-band signalsinto digital data and vice versa. An MES ACSE 54 controls the receiveand transmit frequencies of the radio frequency modulator/demodulator 52and also generates signals for transmission through the radio frequencymodulator/demodulator 52 and receives signals from the radio frequencymodulator/demodulator 52 during call set-up. Data is exchanged betweenthe radio frequency modulator/demodulator 52 and an MES interface 56which provides a voice input/output 58 and a data input/output 60, sothat the MES 6 can be used for voice and data communications. The MESinterface 56, is controlled by a controller 62.

The MES 6 may be a portable unit, for use in the INMARSAT-M™ or theINMARSAT-C™ systems. In the latter, only data communications areavailable and no voice input/output 58 is provided.

Alternatively, the MES 6 may be a fixed installation, connected to alocal network. Hence, the MES 6 need not in fact be mobile.

Further details of the LES 2 and the MES 6 are described in BritishPatent Publications Nos. 2286739 and 2294614 and Applications Nos.9506759.1 and 9512283.4.

A first embodiment of the present invention, as implemented in thesystem described above, will now be described. In this embodiment, asingle return signalling channel is provided for all of the spot beamsreceived by the L-band receive antenna 22. The return intermediatefrequency processor 26 is arranged to convert the single returnsignalling channel, as received in each spot beam, into a correspondingfrequency channel in the return feeder link 4. For example the satellite8 receives the signalling channel at a frequency F in each of the spotbeams 10 a to 10 g, and retransmits the signals received in each of thespot beams in a corresponding set of frequencies F₁ to F₇ in the feederlink.

The global beam 11 generated by the L-band transmit antenna 20continuously transmits signalling channel information received from theLES 2 in a common forward channel.

FIG. 5 shows the application of the first embodiment in a case where theMES 6 initiates a call. Before the start of the call, the LES 2transmits signalling channel information S to the MES 6 in the commonforward channel broadcast by the global beam 11 of the satellite 8. TheMES ACSE 54 receives the signalling channel information and tunes theradio frequency modulator/demodulator 52 to the signalling channelindicated by the signalling channel information S. The MES ACSE 54 thensends a request signal R through the RF modulator/demodulator 52 fortransmission through the antenna 50. The L-band receive antenna 22receives the request signal R with varying signal strengths in each ofthe spot beams, and the return combiner 24 separates the request signalR, as received in each spot beam, into separate channels. The LES ACSE48 monitors the strength of the request signal R, as received in eachspot beam, and determines in which spot beam 10 the request signal R wasmost strongly received. The LES ACSE 48 then selects a communicationschannel which is available and allocated to the selected spot beam, andassigns that communications channel to the MES 6. The LES ACSE 48generates a communications channel instruction C which is broadcast bythe satellite 8 in the common channel in the global beam 11. The MES 6receives the communications channel instruction C and the MES ACSE 54tunes the RF modulator/demodulator 52 to the communications channelrepresented in the communications channel instruction C. Call set-up isthen complete and communications transactions take place from the LES 2to the MES 6 (T₁) and vice versa (T₂) in the communications channelallocated to the MES 6.

In the embodiment shown in FIG. 6, the call is initiated by the LES 2.In this case, the LES 2 transmits, as well as the signalling channelinformation S, a request to communicate Q to the MES 6 in the commonchannel through the global beam 11. The MES 6 responds in the signallingchannel with a ready-to-communicate signal R′. A communications channelis allocated to the MES 6, and communications transactions T₁, T₂ takeplace as in the embodiment shown in FIG. 5.

The first embodiment relies on the ability of the satellite 8 to receivethe same signalling channel in all of the spot beams. However,conventional satellites are designed not to receive the same frequencychannels in overlapping spot beams. Instead, the spot beams are dividedinto three or more groups, labelled as X, Y and Z in FIG. 1. All thespot beams in the same group are allocated the same frequency channels,but because the spot beams within each group do not overlap, there is notransmit or receive interference in these channels.

A second embodiment of the present invention is similar to the firstembodiment, but is applied to circumstances when the satellite 8 dividesspot beams into groups as described above. In this embodiment, adifferent return signalling channel is allocated to each of the groupsX, Y and Z. For example, the satellite 8 receives a first signallingchannel at frequency f₁ from all the group X spot beams, a secondsignalling channel at frequency f₂ from all the group Y spot beams, anda third signalling channel at frequency f₃ from all the group Z spotbeams. Group X comprises Spot beams 10 a, 10 c and 10 e; the firstsignalling channel received from spot beam 10 a is retransmitted atfrequency f₁ in the feeder link 4, the first signalling channel receivedfrom spot beam 10 c is retransmitted at frequency f₂ and the firstsignalling channel received from spot beam 10 e is retransmitted atfrequency f₃. Likewise, the second signalling channel received by spotbeams 10 b, 10 d and 10 f is retransmitted at frequencies f₄, f₅ and f₆on the feeder link 4, and the third signalling channel received by spotbeam 10 g is retransmitted at frequency f₇ in the feeder link 4.

When the MES 6 initiates a call, a protocol exchange takes place asshown in FIG. 7. The LES 2 sends multiple signalling channel informationS_(XYZ) to the MES 6 through the common channel provided in the globalbeam 11. The MES 6 transmits a first request R_(X) on the firstsignalling channel allocated to group X, a second request R_(Y) on thesecond signalling channel allocated to group Y, and a third requestR_(Z) on the third signalling channel allocated to group Z. If, as shownin FIG. 1, the MES 6 is located in a spot beam of group X, the LES 2receives the first request signal R_(X) from the spot beam 10 a with thegreatest signal strength and therefore allocates a communicationschannel to the MES 6 in spot beam 10 a, which is in group X. The LES 2sends a communications channel instruction C_(X) to instruct the MESACSE 54 to tune the RF modulator/demodulator 52 to the allocatedchannel. Communications transactions T₁ and T₂ then take place in theallocated channel.

An example in which a call is initiated by the LES 2 is shown in FIG. 8.The signal transmitted by the LES 2 in the common channel of the globalbeam 11 includes multiple signalling channel information S_(XYZ), as inthe example shown in FIG. 7, as well as a request to communicate Q as inthe example shown in FIG. 6. In the example shown in FIG. 8, the MES 6transmits sequentially ready-to-communicate signals R′_(X), R′_(Y) andR′_(Z) in the signalling channels allocated to groups X, Y and Zrespectively. In this case the MES 6 is located in the spot beam 10 g,which is a group Z spot beam, and so the LES 2 receives the thirdready-to-communicate signal R′_(Z), but not the first twoready-to-communicate signals R′_(X) and R′_(Y). As a result, the LES 2broadcasts a communications channel instruction C_(Z) in the commonchannel, so as to allocate a communications channel to the MES 6 in thespot beam 10 g in which the MES 6 is located, which is in group Z. Thecommunications transactions T₁ and T₂ then proceed in the allocatedcommunications channel.

In a preferred embodiment, the first, second and third requests R_(x),R_(y) and R_(x) and the first, second and third ready to communicatesignals R_(x)′, R_(y)′ and R_(x)′ are sent in different time slotswithin a single frame of a time-division multiplexed (TDM) signallingchannel. Each of the signals has a continuation bit set to indicate tothe LES 2 that all the signals are to be stored at the LES 2 before adecision to allocate a communications channel is made.

If the MES 6 is located close to the boundary of a spot beam, a strongrequest signal R or ready-to-communicate signal R′ may be received inmore than one spot beam. In that case, the signal level of each receivedlevel is measured at the radio frequency demodulator 42 and the LES ACSE48 allocates a communications channel to the MES 6 in a selected one ofthe spot beams with n which a signal with a strength above apredetermined threshold was received. The LES 2 selects one of the spotbeams which has the greatest number of available communicationschannels. Alternatively, the strength of signal received in each of thespot beams is compared, and the spot beam is selected in which thestronger signal was received. Alternatively, one of the spot beams inwhich a signal above a certain level was received may be selected atrandom. The spot beam may be selected by a combination of the abovecriteria. For example, the spot beams may be ranked according to thestrength of signal received in each spot beam, and the highest rankingspot beam in which a communications channel is available may beselected. Alternatively, weights may be calculated for each spot beamaccording to the channel allocations in that spot beam, such that spotbeams having more available capacity are more likely to be chosen.

In a first variant of the second embodiment, the LES 2 transmits thecommunications channel instruction C as soon as a corresponding requestsignal R or ready-to-communicate signal R′ is received with a strengthgreater than a predetermined level. As shown in FIG. 9, the MES 6transmits a request signal R_(x) in the first signalling channel andsets a timer. If no response is received within a predetermined periodt₁ as determined by the timer, the MES 6 transmits a request signalR_(y) in the second signalling channel. In this case a response in theform of the communication channel instruction C_(y) is received from theLES 2 within the period t₁ and the MES 6 is allocated a channel in thespot beam selected by the LES 2. However, if no response were to bereceived to the request signal R_(y) within the period t₁, a furtherrequest signal R_(z) would then be sent to the LES 2 and a responsewould again be awaited.

The transmission of each request signal R_(x), R_(y) or R_(z) may berepeated a predetermined number of times and a response awaited eachtime before changing the request signal to the next signalling channel.

In a second variant of the second embodiment the MES 6 storesinformation on the spot beam group X, Y or Z used for eachcommunication. Before signalling to the LES 2, the MES 6 determineswhether the last communication took place in a spot beam group supportedby the LES 2. If so, the MES 6 transmits the request signal R orready-to-communicate signal R′ in the signalling channel correspondingto the spot beam group used for the last communication. If no responseis received, the MES 6 repeats the signal R or R′ in the same signallingchannel and waits for a response after each repeat. The number ofrepeats may depend on the number of previous communications performedsuccessfully in the corresponding spot beam group, up to a maximumnumber of times. If no response is received even after the predeterminednumber of repeats, the MES 6 reverts to a spot beam group search processas described above in the second embodiment or the first variantthereof.

In a third variant of the second embodiment, the LES 2 sends spot beamidentification information to the MES 6 in a spot beam identificationfield during the communication transaction T₁. The spot beamidentification information determines which signalling channel the MES 6will use during a subsequent call set-up with the LES 2. The spot beamidentification information may indicate a specific spot beam, in whichcase the MES 6 will transmit the request signal R or theready-to-communicate signal R′ in the signalling channel correspondingto that spot beam during subsequent call set-up. The specific spot beammay be the beam currently selected for communication. Alternatively, thespot beam identification information may indicate that the MES 6 shoulduse its own stored information during subsequent call set-up, as in thesecond variant described above. Alternatively, the spot beamidentification information may indicate that no pre-stored informationis to be used during subsequent call set-up, and the MES 6 will transmitin all signalling channels, as shown in FIG. 7 or FIG. 8.

A third embodiment of the present invention is applied to a terrestrialcellular communications system assisted by information broadcast from asatellite. As shown in FIG. 10, the terrestrial cellular systemcomprises a control centre 72 connected co a plurality of base stations70 a to 70 e, each of which is able to transmit and receive signals overan area which defines a cell. A communications link is establishedbetween a mobile terminal 76 and the control centre 72 through the basestation 70 d which defines the cell into which the mobile terminal 76falls. A satellite 74 broadcasts signalling channel information over anarea 78 which encompasses all of the cells. Each co the base stations 70is arranged to receive the same signalling channel. The mobile terminal76 receives information broadcast by the satellite 74 which indicatesthe signalling channel, and transmits a signal on the signallingchannel, which may be received by any of the base stations 70. Thecontrol censure 72 compares the strength of the signal received from themobile terminal 76 in the signalling channel from each of the basestations 70 and thereby determines in which cell the mobile terminal 76is located. The control centre 72 then sends a control signal to themobile terminal 76 through the satellite 74 to indicate whichcommunications channel has been assigned to the mobile terminal 76. Themobile terminal 76 then uses this communications channel for subsequentcommunications.

The third embodiment may be applied to a GSM terrestrial cellularsystem.

In the first and second embodiments described above, both the spot beamidentification protocol exchange and subsequent communication take placethrough the satellite 8. The present invention is also applicable tocombined satellite and terrestrial communications systems in whicheither a satellite link or a terrestrial cellular link is selected forcommunication according to the position or status of the MES 6. In thatcase, the spot beam identification protocol exchange is carried outthrough the satellite 8, in order to locate the MES 6, and subsequentcommunication takes place either in a terrestrial cellularcommunications channel or a satellite communications channel, dependingon the determined location of the MES 6.

The first and second embodiments have been described above withparticular reference to geostationary satellites, but the presentinvention is also applicable to communications systems which usenon-geostationary satellites. If non-geostationary satellites are used,it is likely that the MES 6 will move out of its originally allocatedspot beam during a call. In that case, the MES 6 is handed over to a newcommunications channel in the spot beam which has moved onto the MES 6.In one variant, the MES 6 detects that the quality of reception in thecommunications channel has fallen below a predetermined threshold andinitiates a spot beam identification protocol similar to that describedabove with reference to FIG. 5 or FIG. 7, while the call is still inprogress, and returns to the new communications channel indicated by theLES 2 in the communications channel instruction C.

In another variant, the LES 2 detects that the quality of reception inthe communications channel has fallen below a predetermined thresholdand initiates a spot beam identification protocol similar to thatdescribed above with reference to FIG. 6 or 8, while the call is stillin progress.

If the call is a data call, data communication is temporarilyinterrupted while the handover protocol exchange takes place, oralternatively individual protocol signals are interspersed betweenperiods of data communication. If the call is a voice call, individualprotocol signals may be sent by the LES 2 or the MES 6 during periods inwhich the respective fixed or mobile user is silent. In another variant,the protocol exchange takes place at regular intervals, or during anypauses in voice or data communication, regardless of whether the qualityof communication has fallen below a predetermined threshold, to ensurethat the most suitable available spot beam is used throughout the call.

The present invention is not limited to currently proposed orimplemented INMARSAT™ communications systems, but may be applied toother geostationary or non-geostationary satellite communicationsystems. The functional blocks shown in the diagrams do not necessarilyrepresent discrete circuitry, but several of the block functions may beperformed by a single unit or a single function may be distributedthrough several units.

In the first and second embodiments, a single communications channel isassigned to each carrier frequency; the system is an SCPC (singlechannel per carrier) system. The third embodiment advantageously usesmultiple time-divided channels in each carrier frequency, in a TDMA(time-divided multiple access) system. However, the present invention isnot limited to a specific channel format, but may also be applied tocode-divided multiple access (CDMA) or slotted or non-slotted ALOHAchannel formats, for example.

The allocation of channels to carrier frequencies need not remainconstant throughout a call, but the carrier frequency assigned to eachchannel may vary according to a predetermined sequence; this is known as“frequency-hopping.”

In the first and second embodiments, channel assignment is performed bythe LES 2, and the satellite 8 is controlled by the LES 2. However, thechannel assignment and location detection could alternatively beperformed within the satellite 8, with the satellite 8 generating thesignalling channel information S and communications channel instructionC.

The present invention is applicable both to half-duplex communicationssystems, in which a single channel is used for both forward and returncommunication, and to full duplex communications systems, in whichseparate channels are used for forward and return communication.

What is claimed is:
 1. A method of determining in which of a pluralityof spot beams projected by a satellite a radio frequency transceiver islocated, said method comprising: detecting whether a signal transmittedby said transceiver is received in a signalling channel assigned to allof said plurality of spot beams, and, if so, detecting in which of saidplurality of spot beams the signal was received and selecting one ofsaid spot beams on the basis of said detecting step.
 2. A method asclaimed in claim 1, including, if said signal is received in a set ofmore than one of said plurality of spot beams, selecting one of said setof spot beams on the basis of at least one of: the relative quality withwhich said signal was received in each of the spot beams of at leastsome of said set; the relative number of communications channelsavailable in each of the spot beams of at least some of said set; and arandom or pseudo random selection from at least some of said set.
 3. Amethod of assigning a communications channel comprising performing amethod as claimed in claim 1 or claim 2, selecting a communicationschannel assigned to said selected spot beam, and transmitting acommunications channel assignment command to said transceiver so as toset the transceiver to communicate in said communications channel.
 4. Amethod as claimed in claim 1, including, after said selecting step,transmitting spot beam identification information identifying theselected spot beam to the transceiver.
 5. A method as claimed in claim1, comprising, before the step of receiving said signal, transmitting asignaling channel assignment command to said transceiver so as to setthe signalling channel in which the transceiver transmits said signal.6. A method as claimed in claim 5, wherein the satellite projects aglobal beam which substantially encompasses said plurality of spot beamsand said signaling channel assignment command is transmitted in saidglobal beam.
 7. A method as claimed in claim 6, wherein saidcommunications channel assignment command is transmitted in said globalbeam.
 8. A method as claimed in claim 1, wherein the satellite projectsat least one further spot beam able to receive a further signallingchannel, said method further comprising detecting whether a furthersignal transmitted by said transceiver is received in said furthersignalling channel.
 9. A method as claimed in claim 8, wherein thesatellite projects more than one said further spot beams, furthercomprising, if said further signal is received, determining the locationof said transceiver by detecting in which of said further spot beamssaid further signal is detected, and selecting one of said plurality ofspot beams or further spot beams on the basis of said detection.
 10. Amethod of determining in which of a plurality of spot beams projected bya satellite a radio frequency transceiver is located, said plurality ofspot beams comprising at least first, second and third spot beam groupsable to receive respective first, second and third different signallingchannels, and at least one of said spot beam groups comprising more thanone of said spot beams which are mutually non-overlapping, said methodcomprising: receiving at least one of a first, second or third signaltransmitted respectively on said first, second and third signallingchannels by said radio frequency transceiver, and detecting in which ofsaid plurality of spot beams said at least one of said first, second orthird signal was received, and selecting one of said plurality of spotbeams on the basis of said detecting step.
 11. A method as claimed inclaim 10, wherein said detecting step comprises detecting whether one ofsaid first, second or third signal is received with a quality whichexceeds a predetermined threshold in any one of said spot beams, andsaid selecting step comprises selecting said one spot beam.
 12. A methodas claimed in claim 10, wherein said detecting step comprises detectingwhether said at least one of said first, second or third signal isreceived in a set of greater than one of said plurality of spot beams,and said selecting step comprises selecting one of said set of spotbeams on the basis of at least one of: the relative quality with whichsaid at least one of said first, second or third signal was received inat least some of the spot beams of said set; the availability ofcommunications channels in at least some of the spot beams of said set;and a random selection from at least some of said set.
 13. A method ofassigning a communications channel comprising: performing a method asclaimed in any one of claims 10 to 12, selecting a communicationschannel allocated to said selected spot beam, and transmitting acommunications channel assignment command to said transceiver so as toset the transceiver to communicate in said communications channel.
 14. Amethod as claimed in claim 10, comprising, before said receiving step,transmitting a signalling channel assignment command to said transceiverto as to set said first, second and third signalling channels in whichthe transceiver transmits respectively said first, second or thirdsignals.
 15. A method as claimed in claim 14, wherein the satelliteprojects a global beam which substantially encompasses said plurality ofspot beams and said signaling channel assignment command is transmittedin said global beam.
 16. A method of operating a radio frequencytransceiver for communication via a satellite which projects a pluralityof spot beams, said method comprising: transmitting a signal in asignalling frequency channel to the satellite, the satellite beingarranged to simultaneously receive the signalling frequency channel inmore than one of said spot beams; receiving in a common channel acommunications channel assignment command; and setting said transceiverto communicate in a communications channel selected according to saidcommunications channel assignment command.
 17. A method as claimed inclaim 16, further comprising, before said transmitting step, receivingin said common channel a signalling frequency channel assignmentcommand; and selecting said signalling frequency channel according tosaid signalling frequency channel assignment command.
 18. A method asclaimed in claim 17, wherein said selecting step comprises selectingmore than one signalling frequency channel according to said signallingfrequency channel assignment command, and said transmitting stepcomprises transmitting at least one signal, including said signal,respectively in at least one of said signalling frequency channels. 19.A method as claimed in claim 16, wherein said transmitting stepcomprises transmitting at least one signal, including said signal, in atleast one signalling frequency channel, including said signallingfrequency channel.
 20. A method as claimed in claim 18 or 19, whereinsaid transmitting step comprises transmitting a respective one of saidat least one signal in each of said signalling frequency channels.
 21. Amethod as claimed in claim 20, wherein said respective signals aretransmitted in respective different time slots within a singletime-division multiplexed frame.
 22. A method as claimed in claim 18 or19, wherein said transmitting step comprises sequentially transmitting arespective said signal in different ones of said signalling frequencychannels until said communications channel assignment command isreceived.
 23. A method as claimed in claim 16, wherein the step oftransmitting said signal is repeated a predetermined number of times ifno communications channel assignment command is received in responsethereto.
 24. A method as claimed in claim 23, wherein the predeterminednumber of times is variable and dependent on the result of one or moreprevious communications by the radio frequency transceiver.
 25. A methodas claimed in claim 24, wherein the predetermined maximum number oftimes is dependent on the number of previous successful communicationsby the radio frequency transceiver in one or more of said plurality ofspot beams which are able to receive the signalling frequency channelsin which said signal is transmitted.
 26. A method as claimed in claim16, further comprising, before said transmitting step, selecting saidsignalling frequency channel according to information derived from oneor more previous communications by the radio frequency transceiver. 27.A method as claimed in claim 26, wherein said signalling frequencychannel is selected so as to correspond to one or more of said spotbeams used in said one or more previous communications.
 28. A method asclaimed in claim 26 or 27 wherein said signalling frequency channel isselected according to control information received during said one ormore previous communications.
 29. A method of controlling a satellite,comprising: projecting a plurality of spot beams on earth's surface; andreceiving a same common incoming signaling frequency channel in all ofthe spot beams generated by the satellite.
 30. A method of operating anearth station in a satellite communications system, comprising receivingfrom a satellite a plurality of feeder link channels corresponding to acommon signalling channel received respectively by a plurality of spotbeams generated by said satellite.
 31. Apparatus for determining inwhich of a plurality of spot beams, projected by a satellite, a radiofrequency transceiver is located, the apparatus comprising: detectingmeans for detecting whether a signal transmitted by said transceiver isreceived in a signalling channel assigned to all of said plurality ofspot beams and for detecting in which of said plurality of spot beams asignal transmitted by said transceiver is received; and selecting meansfor selecting one of said spot beams in response to said determiningmeans.
 32. Apparatus as claimed in claim 31, wherein said selectingmeans is arranged to select one of a set of more than one spot beams inwhich the signal is received on the basis of at least one of: therelative quality with which said signal was received in at least some ofthe spot beams of said set; the availability of communications channelsin at least some of the spot beams of said set; and a random or pseudorandom selection from at least some of said set.
 33. Apparatus asclaimed in claim 31 or 32, including means for transmitting spot beamidentification information identifying the selected spot beam to thetransceiver.
 34. The apparatus as claimed in claim 33, wherein theapparatus is included in an earth station.
 35. Apparatus for assigning acommunications channel in a satellite communications system, comprising:apparatus as claimed in claim 31; assigning means for assigning acommunications channel available in said selected spot beam to saidtransceiver; and means for generating a communications channelassignment command for transmission to said transceiver so as to set thetransceiver to communicate in said communications channel.
 36. Theapparatus of claim 35, further comprising: means for generating asignalling channel assignment command for transmission to saidtransceiver so as to set the signalling channel in which the transceiverwill transmit said signal; means for allocating said signalling channelassignment command to a global beam, projected by the satellite, whichsubstantially encompasses said plurality of spot beams; and means forallocating said communications channel assignment command to said globalbeam.
 37. Apparatus as claimed in claim 31, comprising means forgenerating a signalling channel assignment command for transmission tosaid transceiver so as to set the signalling channel in which thetransceiver will transmit said signal.
 38. Apparatus as claimed in claim31, wherein said detecting means is further arranged to detect whether afurther signal transmitted by said transceiver is received in a furthersignalling channel which is receivable in at least one further spot beamprojected by said satellite and said selecting means is arranged toselect one of said plurality of spot beams or said at least one spotbeam in response to said detecting means.
 39. Apparatus as claimed inclaim 38, further comprising means for detecting in which of more thanone said further spot beams said further signal is received. 40.Apparatus for determining in which of a plurality of spot beams,projected by a satellite, a radio frequency transceiver is located,comprising: means for determining whether at least one of a first,second and third signal are received from said transceiver respectivelyon a first, second and third signalling channel, said first, second andthird signalling channels being allocated respectively to first, secondand third groups of said plurality of spot beams, at least one of saidspot beam groups comprising more than one of said spot beams which aremutually non-overlapping; detecting means for detecting in which of saidplurality of spot beams said at least one of said first, second andthird signals was received; and selecting means for selecting one ofsaid plurality of spot beams in response to said detecting means. 41.Apparatus as claimed in claim 40, wherein said selecting means isarranged to select one of said spot beams if a corresponding one of saidfirst, second and third signals is received with a quality which exceedsa predetermined threshold in said one spot beam.
 42. Apparatus asclaimed in claim 40, wherein said selecting means is arranged to selectone of a set of greater than one of said spot beams detected by saiddetecting means on the basis of at least one of: a relative quality withwhich the corresponding one of said first, second and third signals wasreceived in at least some of the spot beams of said set; theavailability of communications channels in at least some of the spotbeams of said set; and a random or pseudo random selection from at leastsome of said set.
 43. Apparatus for assigning a communications channelin a satellite communications system, comprising: apparatus as claimedin any one of claims 40 to 42; means for selecting a communicationschannel allocated to said selected spot beam; and means for generating acommunications channel assignment command for transmission to saidtransceiver so as to set the transceiver to communicate in saidcommunications channel.
 44. The apparatus of claim 43, furthercomprising: means for generating a signalling channel assignment commandfor transmission to said transceiver so as to set said first, saidsecond and said third signalling channels in which the transceiver willtransmit respectively said first, said second and said third signals;means for allocating said signalling channel assignment command to aglobal beam, projected by the satellite, which substantially encompassessaid plurality of spot beams; and means for allocating saidcommunications channel assignment command to said global beam.
 45. Theapparatus as claimed in claim 43, wherein the apparatus is included inan earth station.
 46. Apparatus as claimed in claim 40, comprising meansfor generating a signalling channel assignment command for transmissionto said transceiver so as to set said first, second, and thirdsignalling channels in which the transceiver will transmit respectivelysaid first, second and third signals.
 47. Apparatus as claimed in claim37 or claim 46, including means for allocating said signalling channelassignment command to a global beam, projected by the satellite, whichsubstantially encompasses said plurality of spot beams.
 48. Apparatus asclaimed in claim 47, further including means for allocating saidcommunications channel assignment command to said global beam.
 49. Theapparatus as claimed in claim 49, wherein the apparatus is included inan earth station.
 50. The apparatus as claimed in claim 43, wherein theapparatus is included in a satellite.
 51. The apparatus as claimed inclaim 48, wherein the apparatus is included in an earth station. 52.Apparatus for controlling a radio frequency transceiver forcommunication via a satellite which projects a plurality of spot beams,more than one of which is simultaneously able to receive a samesignaling channel, comprising: generating means for generating a signalfor transmission in said signaling channel; means for receiving in acommon channel a communications channel assignment command; and meansfor setting said transceiver to communicate in a communications channelselected according to said communications channel assignment command.53. Apparatus as claimed in claim 52, further comprising: means forreceiving a signalling channel assignment command; and selecting meansfor selecting said signalling channel according to said signallingchannel assignment command.
 54. Apparatus as claimed in claim 53,wherein said selecting means is arranged to select more than onesignalling channel, including said signalling channel, according to saidsignalling channel assignment command, and said generating means isarranged to generate at least one signal, including said signal,respectively in at least one of said signalling channels.
 55. Apparatusas claimed in claim 52, wherein said generating means is arranged togenerate at least one signal, including said signal, in at least onesignalling channel, including said signalling channel.
 56. Apparatus asclaimed in claim 54 or 55, wherein said generating means is arranged togenerate a respective one of said at least one signal for transmissionin each of said signalling channels.
 57. Apparatus as claimed in claim56, wherein said generating means is arranged to output the respectivesignals in respective different time slots within a single time-divisionmultiplexed frame.
 58. The apparatus as claimed in claim 57, wherein theapparatus is included in an earth station.
 59. The apparatus as claimedin claim 56, wherein the apparatus is included in an earth station. 60.Apparatus as claimed in claim 54 or 55, wherein said generating means isarranged sequentially to output respective ones of said at least onesignal for transmission in different ones of said signalling channelsuntil said communications channel assignment command is received. 61.Apparatus as claimed claim 52, wherein said generating means is arrangedto repeat the signal a predetermined number of times for transmission insaid signalling channel if the communications channel assignment commandis not received in response to the signal.
 62. Apparatus as claimed inclaim 61, including means for storing information derived from one ormore previous communications by the radio frequency transceiver, whereinsaid predetermined number of times is dependent on said information. 63.Apparatus as claimed in claim 62, wherein said predetermined number oftimes is dependent on the number of previous successful communicationsby the radio frequency transceiver in one or more of said plurality ofspot beams which are able to receive the signalling channel in whichsaid signal is transmitted.
 64. Apparatus as claimed in claim 52,including means for selecting said signalling channel according toinformation derived from one or more previous communications by theradio frequency transceiver.
 65. Apparatus as claimed in claim 64,wherein said selecting means is arranged to select said signallingchannel so as to correspond to one or more of said spot beams used insaid one or more previous communications.
 66. Apparatus as claimed inclaim 64 or 65, wherein said selecting means is arranged to select saidsignalling channel according to control information received during saidone or more previous communications.
 67. The apparatus as claimed inclaim 66, wherein the apparatus is included in an earth station.
 68. Achannel assignment apparatus for controlling a satellite which isarranged to project a plurality of spot beams on a surface of the earth,said apparatus being arranged to assign a same common signaling channelto all of the spot beams projected by the satellite.
 69. Apparatus forreceiving from a satellite a different one of a plurality of feeder linkchannels corresponding to a same common signaling channel receivedrespectively by each of a plurality of spot beams projected by saidsatellite.
 70. The apparatus as claimed in any one of claims 31 to 32,35 to 42, 47, 57, to 55, 61 to 65 or 67 to 68, wherein the apparatus isincluded in an earth station.
 71. The apparatus as claimed in any one ofclaims 31 to 42, 68 or 69, wherein the apparatus is included in asatellite.
 72. A method of selecting one of a plurality of cells forcommunication with a radio frequency transceiver in a cellularcommunications system, each of said cells including a base station forcommunication with radio frequency transceivers within said cell, saidmethod comprising transmitting signalling channel information in abroadcast channel via satellite over said plurality of cells, saidsignalling channel information designating a signalling channel;receiving said signalling channel in each of said plurality of cells,including receiving a signal from a radio frequency transceiver in saidsignalling channel; and selecting one of said plurality of cells inwhich said signal was received, for communication with said radiofrequency transceiver.
 73. A method as claimed in claim 72, wherein saidone cell is selected on the basis of at least one of a quality of saidsignal received in each of the cells, the availability of communicationschannels in those cells in which the signal was received, and a randomor pseudo random selection from those cells in which the signal wasreceived.
 74. A method as claimed in claim 72 or claim 73, furthercomprising broadcasting a communications channel assignment command tosaid transceiver so as to set the transceiver to communicate in saidcommunications channel.
 75. Apparatus for selecting one of a pluralityof cells for communication with a radio frequency transceiver in acellular communications system, each of said cells including a basestation for communication with radio frequency transceivers within saidcell, said apparatus comprising: means for outputting signalling channelinformation for broadcasting via satellite over said plurality of cells,said signalling channel information designating a signalling channel;means for receiving a signal in a signalling channel assigned to each ofsaid plurality of cells; and selecting means for selecting one of saidplurality of cells in which said signal was received for communicationwith said radio frequency transceiver.
 76. Apparatus as claimed in claim75, wherein said selecting means is arranged to select said one cell onthe basis of at least one of a quality of said signal received in eachof the cells, the availability of communications channels in those ofthe cells in which the signal was received, and a random or pseudorandom selection from those cells in which the signal was received. 77.Apparatus as claimed in claim 75 or claim 76, further comprising meansfor outputting a communications channel assignment command fortransmission in said broadcast channel to said transceiver so as to setthe transceiver to communicate in said communications channel.